For with Mario -- A trick that takes it from needing super perfect timing, to just a little timing:
You're on the right track with just getting really good spacing and timing, and adding an air-spin after springing off that last drybones. This can work, but needs almost impossible spacing and timing.
To make getting the coin manageable -- here's the trick:
Doing a mid-air spin in the air, before springing off an enemy, (the last drybones in this case) adds just a little more oomph to the spring off the enemy.
It still needs some finesse, but now I can get it most times, instead of the super perfect timing and such that I needed before adding the extra air-spin before springing off the last drybones.
With this way also -- I don't think I worry about springing on all the drybones, but at least off the 2nd and 3rd. I don't know if this matters either -- but I start from under the question blocks, not above. I feel like this process is most manageable, and maybe coming from the lower ground makes less chance of missing the coin by bouncing off the cieling when the spring off the last drybones is a lil high.